Hot pressing to form canned uranium slugs



July 25, 1961 s. B. ROBOFF ET AL HOT PRESSING TO FORM CANNED URANIUMsums Filed Oct. 11, 1951 RS NGSTON OBOFF INVENTO WALTER E. KI SJANLEY B.R

United States Patent 2,993,786 HOT PRESSING TO FORM CANNED URANIUM SLUGSStanley B. Robofi, White Plains, and Walter E. Kingston,

Bayside, N.Y., assignors, by direct and mesne assignments, to the UnitedStates of America as represented by the United States Atomic EnergyCommission Filed Oct. 1 1, 1951, Ser. No. 250,822 6 Claims. (Cl. 75208)This invention relates to a method of and apparatus for simultaneouslypreparing and canning metallic slugs or billets. Mor particularly, itrelates to a method and apparatus for preparing canned billets or slugsof metals such as uranium, thorium, protectinium, plutonium, berylliumand zirconium.

In accordance with prior practice, the metal such as uranium which is tobe canned is first melted, then cast into ingots which are subsequentlyhot and cold rolled or extruded to form billets. These billets are thencut up into desired lengths, in preparation for canning. Prior tocanning, the cut billets are dipped in a low melting point solder, oralloy Whose purpose is to promote good bonding between the uranium slugand the enclosing aluminum can and to prevent rapid diffusion of theuranium through the aluminum can. The billet is then sealed in analuminum can.

This process as presently practiced, is costly. It also requiresinvolved techniques and becomes complicated because of the rigidcontrols if satisfactory results are to be obtained. Furthermore, due'tothe fact that the cast ingots are hot and cold rolled and extruded,'thecrystalline structure of the material takes on a preferred orientationas well as a tendency toward excessive grain growth. Both of thesefactors are believed to contribT- ute to dimensional instability whenthe billets are subjected to certain physical conditions such as thermalcycling, neutron bombardment, etc.

It is, therefore, an object of this invention to improve the techniqueof preparing canned billets or slugs in such 'manner as to make theprocess more economical as well as to improve the product obtained.

It is a further object of this invention to develop a method ofpreparing canned billets or slugs of metal whose dimensions andmetallurgical structure under thermal cycling and radiation will bestable.

2,993,786 Patented July 25,

carrying out preferred embodiments of this invention in which powderedmaterial can be compressed and heated 10 represents an externally heateddie. The die 10 is so designed that a casing or can 12 will form a closefit within the cylindrical aperture of the main body of the die. Thepowder 14 is shown compacted therein between end caps 11 and '13 of thesame material as the casing. The compacted powdered particles 14 arethereby wholly enclosed within the cylindrical casing 12 and the endcaps 11 and 13. Pressure rams 20 and 22 are so designed as to slidesnugly within the casing 12 and are adapted for compacting powderedparticles within the casing 12 as well as for exerting the necessarypressures on the powdered particles '14 through end caps 11 and 13 atsintering conditions.

A water cooled induction coil 24 surrounding the die .10 is shown in thedrawing as a means of supplying heat to both the material beingcompacted and/or sintered and the enclosing metal casing. Any otherheating means designed to provide sufficient heat within the area of thecompacted powdered metal to permit sintering of the particles would ofcourse be satisfactory. This means need not necessarily be limited toeither electric or electronic means.

In FIGURE 3 of the drawings a modified apparatus is shown in which thedie 30 is cup shaped. In this embodiment, the metal casing 32 for thepowder, instead of being an open ended cylinder, is provided with abottom 34 and is only open at one end 36. A single pressure ram 38designed to fit snugly within the cylindrical wall of the metal casing32 is adapted to cooperate with the die member 30 in bringing pressureto bear on powdered It is a further object of this invention to developa method of preparing billets or slugs of metal approaching theoreticaldensity having small grain size and exhibiting a random orientation ofparticles.

It has been found that these objects and other advantages can beobtained by compacting and pressing pow- .dered uranium, zirconium,thorium, protactinium, plutonium, beryllium, or their hydrides undercontrolled temperature conditions.

l -In the drawings which illustrate a preferred embodi mentof apparatussuitable for carrying out the inven- 7 tion, a

FIGURE 1 is a schematic drawing of an apparatus showing the die body,the metal from which the can is formed, the powdered material undercompression, rams for applying pressure, heating means, and means formaintaining vacuum or other atmosphere around the ob- .ject.

FIGURE 2 is an elevation of a sintered canned slug as produced withapparatus of the type shown in FIG- URE 1 when operating in accordancewith this invention.

FIGURE 3 is an elevation partly in section of a modified apparatus inwhich pressure in brought to bear by means of a single pressure ram.

. In .the drawings illustrating a suitable apparatus for particles 31within casing 32 through cap member 39 which is made of the same metalas casing 32. A heating means consisting of electrical resistanceelements 40 is shown as being imbedded in the die 30.

In operation, the casing 12 is placed in the main body of the die, andthe bottom ram 20 on the top of whichris placed cap member 11 is slidinto the bottom end of the casing 12. The proper amount of powder isthen poured into the casing 12 within the die and'leveled off. The topcap member 13 is then placed on the powder and the top ram 22 slid downinside the casing, making contact with the top cap '13. In those casesin which atmospheric conditions, other than those normally encountered,are desired, these conditions are established. After this the powderedparticles may, if desired, be first compacted by means of pressureexerted, by the cooperative action of 'the'rams 20 and 22 and die 10 andthen heated at desired pressures by heating the compacted powderedparticles with the aid of the water cooled induction coil 40 or othersuitable means while maintaining the powdered metalunder the desiredpressure by means of pressure rams 20 and 22. The compacting and heatingoperation may of course be carried out substantially simultaneously.During the hot pressing operation or after the completion of theapplication of pressure, it is possible to fuse the cap ends 11 and 13to the casing material 12 and thus form a clug of compressed cannedsintered material, such as shown in FIGURE 2 of the drawings.

The hot pressing method herein above described has broad application inthe production of many compressed powdered materials, as for example inthe production of multilayer billets which can be drawn into coated orDumet type wires. It is particularly advantageous for the production ofcanned uranium, thorium, plutonium, zirconium, protactinium, orberyllium billets and slugs.

When this process is applied to the manufacture of canned uranium slugs,powdered uranium or uranium hydride powder or alloy powders of uranium(to which may be added if desired other elements or compounds 1 2? .5114? 5W1? growth i ibit r thorium oxide, rcomum oxide, aluminum oxide,beryllium oxide, or silicon dioxide) are introduced into the main bodyof the die and into the casing '12 in a desired amount, iwhereuponthepowder is leveled off and a cap is placed there- .over. After the topplunger has slid down inside the casing to make contact with the topcap, the -pressing operat on is carried out simultaneously with aheating operation. It should be understood that during this operationthe die may be maintained at an optimum temperature or heat may beapplied during the pressing and s 1 ntering operation. This operationmay be carried out in a vacuum or a protective atmosphere of argon,helium, or other gas, and this atmosphere may beestablished prior to orduring the pressing operation by enclosing the die body within a chamberin which the desired atmospheric conditions are maintained. The cannedpowder is heated to the proper temperature either by the high frequencymeans illustrated or by other conventional methods suitable foraccomplishing a similar purpose. After or during the attainment of theoptimum temperature of operation, the correct pressure and timerelationship is established to give the optimum density and otherphysical properties such as grain size, orientation, etc., and toachieve adequate sealing of the end caps to the casing as well as thedesired bonding between the body and the casing. While pressures ofthe-order of'l00 t.s.i. may be used densities approaching theoreticalhave been obtained at pressures of the order of 30 t.s.i. for about oneminute.

To achieve random orientation and small grain size of particles whichwill give dimensional stability under irradiation to the product, it isessential to carry out the hot pressing operation at a temperaturewithin the alpha .phase of the uranium particles, advantageously at atemperature of 450 to 660 C., preferably at about 600 C. At thesetemperatures, the fine grain size and optimum degree of randomness ofgrain orientation can be maintained; strain hardening is avoided anddensity approaching theoretical density can be reached. Since .thesintering and hot pressing are done at temperatures above that at whichstrain hardening can take place in the alpha phase, there will be nocold working and thus there will be no major internal stresses whichwould tend to deform the shapes during use. The original randomness oforientation of the metal or alloy particles v will be retained by thegrains formed from these particles during sintering in the alpha phase.

In the case of plutonium, in order to achieve high density with randomorientation and small grain size of particles, it is essential to carryout this operation at a temperature within the alpha phase oftheplutonium particles, that is, at a temperature of about 75 C. toabout 114 C., preferably at about 110 C.

It is of course understandable that the canning process herein describedmay be used in carrying out operations requiring higher temperatureswhen it may be delsirable, for example, to hot press the particles ineither the beta or gamma phase.

When canning slugs of thorium, beryllium, and zirconium it is practicalto hot press and can at tempera- -tures of at least l000 C., and withprotactinium, at about 300 C. or above. Useful materials for-the casingto envelop these metals may be selected from a variety ,of metals andalloys such as, for example, aluminum, copper, brass, titanium,beryllium and Zirconium; The casing metal will be selected so that itdoes not melt under the conditions used, and isdifferentfrgm the metalof the slug.

p The casing or can in which the metallicparticles are to be enclosedmay be made of any suitablemetal which -will achieve the resultsdesired. In those cases in which ;the method of this invention is to beused for canning uranium it has been found preferable to' make use ofmet or l y ha n high, os on r jstanc ra d tq good bonding propertiessuch as aluminum, zirconium, 75

or beryllium. In .the preferred embodiment of this invention describedherein, aluminum is referred to as the casing material. When aluminum isso used in forming the canned sintered slug of uranium it has been foundthat a certain alloying takes place between the aluminum casing and theuranium sintered body which gives a tenacious alloy type bond with goodthermal conductive properties. This eliminates the need for any bondingagents and furthermore assures for the most effective use of the spacewithin the can and the maximum amount of the sintered uranium body. Thealuminum end caps are likewise alloyed to the uranium body and also tothe aluminum casing, thereby forming the protective covering desired. Ifsuch an alloy bond is not deemed to be adequate or desirable, it is, ofcourse, understood that the casing andend caps can be coated withspecial materials or alloys on the inside in order to achieve maximumprotection. Thus, it would be possible to lead coat the inside of thealuminum casing and end caps or use other methods of furnishingadditional protection.

When preparing canned uranium slugs in accordance with this invention,such slugs will be found to have good dimensional stability, excellentprotective coating, fine grain size, and optimum randomness of grainorientation. Furthermore, it is believed that this method will beconsiderably more economical than any process involving casting ofuranium, hot rolling, annealing, grinding, canning, etc., as is nowcommonly done in the preparation of fuel elements for nuclear reactors.Detailed information concerning the operation of nuclear reactors isgiven in an application of Fermi and Szilard, Serial Number 568,904 nowUnited States Letters Patent No. 2,708,656, issued December 19, 1944.

When preparing canned slugs or shapes in accordance with this invention,it will be found that there will be a saving of metal, sometimesprecious, since this invention assures virtually usage of the metal. Itwill also be found that the material has been formed into the exactconfiguration desired. This is in contrast to other methods of formingthese slugs or shapes which involve the steps of semi-precision castingof ingots, with melting and cropping losses; subsequent extrusion orrolling, with cropping losses; and subsequent machining to size, withinherent machining losses. The prior methods of forming the slugs andshapes are also limited in the economical production of unusualconfigurations.

When preparing canned slugs in accordance with this invention, it willbe found that an excellent heat transfer interface has been readilyformed between the case and the core, eliminating the necessity forspecial bonding materials and laborious bonding procedures which arecurrently used to ensure the existence of a good heat transferinterface. This new method jackets and forms a good interface in oneoperation, without the use of added materials, whereas other methodsrequire extra materials and steps to form a bond between the core andthe jacket.

It is to be understood that if it is desirable to obtain a slug ofuranium or plutonium without the protective metallic can, it will bepossible to make such slug by the same technique, thus obtaining all theadvantages outlined above which are not in part attributable to theprotective metallic coating. In this case, the powder would be pressedin the main body of the die under controlled temperature conditions sothat a fine grain size and optimum degree of randomness of grainorientation is obtained. The technique used is one of hot pressing whichis advantageously done at a temperature above that at which strainhardening can take place in the alpha phase, the preferred temperaturefor uranium,

as indicated previously, being in the neighborhood of herewith disclosepreferred and practical embodiments of the method and apparatus of thisinvention it will be understood that the specific details ofconstruction and arrangement of parts as shown and described are by wayof illustration and are not to be construed as limiting the scope of theinvention.

What is claimed is:

1. In the method of preparing uranium slugs having a density approachingtheoretical density, small grain size and random orientation ofparticles, the steps comprising introducing powdered uranium into a dieand subjecting the powdered material to pressures of 30 to 100 t.s.i.while maintaining the material at a temperature within the range of 450to 660 C.

2. In the method of preparing uranium slugs having a density approachingtheoretical density, small grain size and random orientation ofparticles, the steps comprising introducing powdered uranium into a dieand subjecting the powdered material to a pressure of the order of 30t.s.i. while maintaining the materials at a temperature within the rangeof 450 to 660 C. for a period of approximately 1 minute.

3. In the method of preparing canned uranium slugs the steps comprisingintroducing an open ended can into a snugly fitting die, introducing ameasured quantity of powdered uranium material into said can, placing ametallic disc thereupon, and subjecting the powdered particles to apressure above 30 t.s.i. while maintaining the material at a temperatureof 450 to 660 C.

4. In the method of preparing canned uranium slugs the steps comprisingintroducing a metal sleeve into a die, inserting a disc of metal whichsnugly fits into said sleeve, and resting on a ram projecting into saidsleeve,

6 introducing a measured quantity of powdered uranium material into saidsleeve, placing a metallic disc thereover, and subjecting the powderedparticles to a pressure above 30 t.s.i. while maintaining the materialat a temperature of 450 to 660 C.

5. In the method of preparing canned uranium slugs the steps comprisingintroducing a zirconium sleeve into a die, inserting a disc of zirconiumwhich snugly fits into said sleeve and rests on a ram projecting intosaid sleeve,

introducing powdered uranium material into said sleeve,

placing a zirconium disc thereover, and subjecting the powderedparticles to a pressure above 30 t.s.i. while maintaining the materialat a temperature of 450 to 6. In the method of preparing canned uraniumslugs the steps comprising introducing a metal sleeve into a dieinserting a disc of metal which snugly fits into said sleeve, and restson a ram projecting into said sleeve, introducing powdered uraniummaterial into said sleeve, placing a metallic disc thereover, andsubjecting the powdered particles to a pressure about 30 t.s.i. whilemaintaining the material at a temperature of about 600 C. for a periodof approximately 1 minute.

References Cited in the file of this patent UNITED STATES PATENTS1,670,463 Marden May 22, 1928 1,904,568 Taylor Apr. 18, 1933 2,123,416Graham July 12, 1938 2,907,705 Blainey Oct. 6, 1959 2,934,482 BrooksApr. 26, 1960

1. IN THE METHOD OF PREPARING URANIUM SLUGS HAVING A DENSITY APPROACHINGTHEORETICAL DENSITY, SMALL GRAIN SIZE AND RANDOM ORIENTATION OFPARTICLES, THE STEPS COMPRISING INTRODUCING POWDERED URANIUM INTO A DIEAND SUB-